Not Applicable.
Not Applicable.
The present invention pertains to non-contact measurement of fluid viscosity by use of a miniature laser interferometer.
The invention is a new process for measuring the viscosity of fluids without making any physical contact with the fluid. Viscosity is a manifestation of fluid friction. The standard method for measuring viscosity was invented by the French physician Jean Louis Poiseuille (1799-1869) who devised the first flow viscometer. In the past two centuries many other eminent scientists, including James Clerk Maxwell, have tried their hands at inventing alternative viscometers. In principle all the current methods (Couette, falling-sphere, oscillating disk, and cone-and-plate viscometers) exploit the resistance experienced by an object in contact with and in relative motion to the fluid. While Stokes (1819-1903), father of modern fluid dynamics, recognized that the attenuation of surface waves could be exploited to measure viscosity without the use of a foreign object, no reliable method has been available to date.
U.S. Pat. No. 5,303,030 to Abraham et al. describes use of an optical interferometer for determination of surface tension of a fluid by reflecting a light beam off the surface of a fluid on which capillary waves have been induced. An optical fiber and a capillary wave inducing blade are held in a parallel fixed relation above the fluid surface. The capillary wavelength is determined by measuring the phase difference of the reflected signal at two points on the surface. The wavelength is then used to obtain the surface tension.
The determination of viscosity from the damping of capillary waves has been of great interest as it affords the possibility of a non-contact method for measuring viscosity. The instant invention provides a non-contact method and apparatus for precision measurement of the amplitude, wavelength, and attenuation of capillary waves on fluids. Surface waves on fluids, with wavelengths in the millimeter range, are known as capillary waves. In this wave regime surface tension and viscosity govern the propagation and attenuation of the surface waves while gravity plays a minor role. Therefore, data on dispersion and attenuation of capillary waves may be used to determine the surface tension and viscosity of fluids. Of particular interest has been the determination of viscosity from the damping of surface waves.
To obtain the wave profile of capillary waves the invention employs a fiber-optic detection system that functions as a miniature laser interferometer. The heart of the system consists of a single mode optical fiber, one end of which is positioned a short distance above the fluid surface. Laser light traveling through the optical fiber is partially reflected from the cleaved tip of the fiber and again from the fluid surface. The two reflected beams travel back through the same fiber forming an interference pattern. As the water level changes due to wave motion, the interference signal portrays an accurate record, in real time, of the variation of the gap between the end of the fiber optic cable and the fluid surface.
The invention obtains the wave profile with a resolution of about ten nanometersxe2x80x94some fifty times better than the resolution of a typical optical microscope. The invention has been used to obtain the dispersion and attenuation of capillary waves on pure water as a test case. Furthermore, the attenuation data has been used to obtain the viscosity of pure water as a function of temperature. Results using the invention are consistent with accepted values obtained by traditional flow viscometry thus demonstrating the great utility of this non-contact method for measuring viscosity.
The capillary waves are generated electronically by placing a metallic blade a few tenths of a millimeter above the water surface. A sinusoidal voltage of several hundred volts at a known frequency is applied between the blade and the water. Since water molecules are polar, the alternating electric field under the blade generates two capillary wave trains that recede from the blade on the two sides. Typically the amplitude of these waves is of the order of one micrometer.
To obtain the dispersion data, a standing wave is established on the fluid surface. A standing wave is generated when two blades, separated by a few centimeters, are used to generate waves of the same phase, amplitude, and frequency. Since each blade sends a wave train toward the other, a standing capillary wave is established on the water surface between the two blades. If the distance between the two blades is chosen to be a half odd-integer wavelength, the two wave trains interfere destructively on the outer sides of the blades. This judicious choice of the blades"" separation produces a region of standing waves between the blades while the surface outside the blades remains calm. Measurement of the distance between nodes of the standing waves yields the wavelength of the capillary wave.
In the instant invention, the fiber optic probe is attached to a micro-positioner, which in turn is equipped with a digital micrometer. This enables measurement of the wavelength of the standing capillary waves routinely to within a micrometer. A precise dispersion relation is obtained by a measurement of the wavelength at various frequencies.
To obtain the attenuation data, a traveling wave is established on the fluid surface by using one blade. One fiber probe, held stationery on one side of the blade, provides the reference amplitude. The second fiber probe measures the amplitude of the wave as a function of distance away from the blade. The normalized amplitude attenuation data yields the viscosity of the fluid.
It is an object of the invention to provide a non-contact method and apparatus to precisely measure viscosity of a fluid.
It is a further object of the invention to provide a miniature laser interferometer system which may be used without mechanical contact with the fluid to determine the wave profile of capillary waves upon the surface of the fluid.
It is a further object of the invention to map the wave profile of a traveling capillary wave generated on a fluid surface.
It is yet another object of the invention to create a standing wave on a fluid surface which may be mapped by use of a laser interferometer.
It is still a further object of the invention to provide apparatus for non-contact measurement of the wave-length of a standing wave of varying frequencies generated on the surface of a fluid.
It is a further object of the invention to provide apparatus and a method to measure viscosity of a fluid without risk of contamination of the fluid under examination.
These and other objects of the invention will become apparent from examination of the description and claims which follow.